Percussion waves, sometimes referred to as mechanical waves, are waves which are passed through a medium, for example, water, air, etc., by way of generating a disturbance in the medium that is propagated therethrough because the medium has elastic properties.
Electrostatic transducers for generating and/or sensing percussion waves are well known in the art. Examples are illustrated in U.S. Pat. No. 4,081,626 to Muggli et al. and U.S. Pat. No. 4,695,986 to Hossack. In such transducers, a thin (often 5-10 micrometers in thickness) plastic film, which is metallized on one surface to produce an electrode, is stretched to form a diaphragm over a relatively massive metallic electrode, hereinafter termed the backplate, with the nonconductive surface of the film in contact with the backplate. The metallized surface of the film is separated by way of the insulating film from the electrode backplate so that a capacitor configuration is defined. Further, in order to provide fluid gaps for movement of the electrode diaphragm, the metal surface of the electrode backplate is textured or toughened by sanding, machining, coining, or electric discharge techniques.
In operation, when an alternating current (AC) electrical signal is superimposed on a direct current (DC) voltage bias across the aforementioned electrodes during a transmission mode of operation, the metallized film is stressed and oscillatory formations develop, thereby causing a wave front to be propagated from the film to the adjacent medium, such as water, air, etc. During a receive mode of operation, variable pressure on the diaphragm moves the film, producing a variable voltage across the electrodes which can be sensed.
The surface characteristics of the electrode backplate determine the frequency range and sensitivity of the transducer. With a very smooth, high polished surface, the frequency range can extend to about 500 kilohertz (kHz) although the sensitivity is rather low. With a surface roughened by sandblasting or other methods, or provided with grooves, the sensitivity is higher, but the upper frequency limit is lower.
Electrostatic transducers can be used for a wide variety of applications. They are currently used to stimulate and detect acoustic resonances inside chambers. Determination of certain resonance frequencies is sufficient to obtain gas phase thermophysical properties. Electrostatic transducers can also be used in industrial applications, such as flow metering, pipeline inspection, automated welding, and vehicle guidance.
While transducers constructed in accordance with the foregoing architecture provide suitable operation for many applications, they are not well suited for harsh, high temperature, and/or high pressure environments. At temperatures above 473 Kelvin (K), when exposed to certain compounds, or when exposed to certain radiation, the metallized polymer film will chemically and physically degrade. Polymers adsorb and outgas many other molecular species that contaminate any other fluid under test. Furthermore, the polymers in the films accumulate static electrical charges that render the transducer inoperative. In essence, the polymers act as an electret. In fact some systems have been developed to discharge these films. Finally, because of the manner in which the metal surface of the electrode backplate is typically textured, sharp edges exist and these sharp edges magnify the surrounding electric field, thereby creating sparks and eventually device breakdown.
Electrostatic transducers for harsh, high temperature, and/or high pressure applications are also difficult and expensive to produce on a mass commercial scale. For example, U.S. Pat. No. 4,081,626 to Muggli et al. describes an electrostatic transducer having a metallized film (metal on dielectric Kapton polymer) disposed over an electrode backplate having square groove projections for supporting the metallized film. In order to produce the square groove projections in the electrode backplate, an expensive metal working or coining process and machine must be utilized. This requirement makes this fabrication process and transducer undesirably expensive, complicated, and prohibitive in many circumstances.
As another example, consider U.S. Pat. No. 4,695,986 to Hossack. The foregoing patent describes an ultrasonic transducer also having a metallized polymer (metal on Kapton polymer) film disposed over an electrode backplate and supported by metallic protrusions extending from the electrode backplate. Although the transducer in the Hossack patent is easier to produce than the electrostatic transducer the Muggli patent, the Hossack transducer requires use of an electrochemical machining process which generates huge amounts of toxic waste. Hence, this process results in unnecessary and undesirable expense relative to disposing of the toxic wastes, and the problem is compounded as production requirements are increased.
Hence, a heretofore unaddressed need exists in the industry for an electrostatic transducer which is well suited for harsh, extreme temperature, and/or extreme pressure applications, which does not accumulate static charge or created sparks, which does not suffer from polymer decomposition or degradation, and which is easily and inexpensively manufactured on a mass commercial scale.